Add support for S3 through the REST API

This is currently using the old S3 authentication
(https://s3.amazonaws.com/doc/s3-developer-guide/RESTAuthentication.html),
in order to support Exoscale Storage

Refs #112

config.yml.example.s3

@@ -0,0 +1,31 @@
+development: &defaults
+  maintenance: false
+  # # uncomment this section
+  # s3:
+  #   endpoint: "https://some-endpoint"
+  #   region: "region"
+  #   access_key_id: ""
+  #   secret_key_id: ""
+  #   bucket: "test-bucket"
+  # # Redis is needed for the swift backend
+  # redis:
+  #   host: localhost
+  #   port: 6379
+
+test:
+  <<: *defaults
+  s3:
+    endpoint: "https://some-endpoint"
+    region: "region"
+    access_key_id: ""
+    secret_key_id: ""
+    bucket: "test-bucket"
+  redis:
+    host: localhost
+    port: 6379
+
+staging:
+  <<: *defaults
+
+production:
+  <<: *defaults